Broadly, gasification is the creation of combustible gas known as synthesis gas and commonly referred to as “syngas” herein, from carbon-containing fuels. Gasification is a well-known industrial process used for converting solid, liquid and gaseous feedstocks using reactants such as air, oxygen, and steam into gases such as hydrogen, carbon monoxide, carbon dioxide, and methane. The resulting gases can be used for generating electrical power, producing heat and steam, or as a feedstock for the production of various chemicals and liquid fuels, or any combination of the above. To appreciate the present invention, it will be helpful to have an understanding of the history of gasification and current limitations of prior art in commercial gasification technologies.
Various forms of gasification have been used since the 1800's as a way to convert solid fuels (primarily coal) to gaseous fuel (synthesis gas or syngas). Large-scale coal gasification was first applied as a way to generate town-gas, distributed through pipeline systems throughout much of Europe prior to the development and use of natural gas.
The oldest gasification processes is known as fixed bed (or moving bed) gasification. The Germans originally developed fixed bed gasification and subsequently the first large-scale fixed bed coal gasifiers. Additional processes including fluidized bed gasification and entrained bed or entrained flow gasification processes have been developed more recently by others. All three gasification processes have both beneficial as well as limiting features, which the following discussion will generally describe.
Fixed-bed gasification requires coarse fuels (typically ¼″ to 2″ in diameter). The most well known and widely used fixed-bed gasification technology is the Lurgi Dry Ash gasifier (the ash is recovered dry and does not turn molten or slag). The Lurgi Dry Ash technology is presently used by the country of South Africa, to convert large reserves of coal into syngas, which is then converted to liquid fuels and chemicals via downstream syngas catalysts. Nearly half of all transportation fuels used in South Africa come from the Lurgi-based coal gasification plants in that country. Presently, there is one Lurgi-based gasification installation in the United States that converts lignite (high moisture, low rank) coals to syngas, which is then subsequently converted to pipeline quality methane and sold into the natural gas pipeline system. An advantage of the Lurgi gasifier is its ability to produce a syngas with a ratio of hydrogen to carbon monoxide greater than two. This allows the syngas, after cleaning, to be used in downstream conversion processes to make liquid fuels and chemicals that require this higher ratio.
Fixed-bed gasification uses coarse fuels, which first must be devolatilized (i.e. drive off all volatile compounds) before gasification of the remaining fixed carbon occurs. The carry over in the syngas of tars and oils from the Lurgi Dry Ash gasifier led to the development of the British Gas Lurgi (BG/L) fixed-bed slagging gasifier. While it too carries over tars and oils with the initial raw syngas effluent, the BG/L has the ability to re-circulate and re-inject the tars and oils into the hotter lower slagging (molten ash) section of the gasifier, where the tars and oils are converted into syngas constituents. The reinjection of tars and oils is an unnecessary additional step that is eliminated by the present invention. It would be an advancement in the art of fixed-bed gasifiers to provide a gasification system that prevents the creation of tars and oils in the raw syngas effluent.
Limiting technical features of fixed-bed gasification include: (1) tar and oil carry over with the syngas; (2) difficulty in using coal/fuel fines because they clog the void space between the coarse fuels in the fixed bed (because of this, large piles of discarded coal fines are stockpiled adjacent to these plants or transported off-site to combustion facilities that can use the fines, resulting in low overall coal-to-product efficiencies); (3) difficulty in using liquid hydrocarbon feedstocks; and (4) difficulty in using caking coals (coals with low ash fusion temperatures) which often require mechanical stirring to agitate the fuel bed.
A newer gasification process is known as entrained-flow or entrained-bed gasification. Pulverized solid fuels and/or viscous liquid fuels are fed to the gasification reactor and are rapidly converted to syngas. One major advantage of the entrained-flow gasification processes is that no oils and tars are produced, precluding their presence in the effluent syngas.
Another advantage of the entrained-flow gasification process is its ability to gasify liquid feedstocks (i.e. oil or heavy oil residuals from refineries). The primary entrained-flow gasification technologies include those developed by: Texaco, Shell and Dow Chemical (Destec). Entrained-flow gasification processes are unable to easily process coarse fuels. This makes it difficult to process certain fuels, such as biomass and segregated municipal solid waste or scrap tires (fuels that cannot be economically pulverized for use as fuel). Another limitation of entrained-flow gasification is the inability to achieve wide internal control of the hydrogen to carbon monoxide ratio in the exiting syngas. To adjust this ratio in favor of hydrogen would require an additional downstream water gas shift reactor to increase hydrogen and reduce carbon monoxide.
Entrained-flow gasification technologies also have technical and fuel processing limitations. In order to convey the fuel into the gasifier, some entrained flow gasifiers use a slurry feed that is often water mixed with pulverized solids at approximately a 1:2 ratio respectively. Other commercial entrained flow gasifier use dry feeds pulverized solid into the gasifier. All these gasifiers produce a syngas with a hydrogen to carbon monoxide ratio of approximately 1:1 or less with limited ability to control this ratio. Gasifiers using the slurry feed process for solids feedstocks limit their ability to use high moisture fuels such as lignite coals as too much moisture fed to the gasifier system results in poor gasification performance.
In addition to the limitations cited above, other prior art problems include: (a) clogging, caking and/or undesired accumulations of material in the fuel delivery, bed and/or slag discharge regions of the gasifier; (b) inadequate sulfur and pollutant species control requiring the need for extensive syngas cleaning and processing equipment downstream of the gasifier; (c) high installation and operational costs; and (d) limited fuel flexibility.
A third gasification process known as the fluidized bed process has been utilized on a limited basis. Fluidized bed gasifiers also use coarse fuels, however the coarse fuels are somewhat smaller in size than coarse fuels used by fixed bed gasifiers. An advantage of the fluidized bed processes is its ability to use fuels with relatively high ash contents. Fluidized bed gasification processes face challenges related to fuel agglomeration in the bed. In addition, fluidized bed gasification processes realize some similar limitations as fixed bed processes.
To summarize, the principle limitations of prior gasification art include the limited ability to simultaneously process or gasify both coarse and fine solid fuels or simultaneously process or gasify both coarse and liquid hydrocarbon feedstocks resulting in a general lack of fuel flexibility in any one system. While gasification plants are generally able to meet exceedingly high emission limitations, their capital costs remain high. Costs in controlling pollutant species must be reduced to allow its widespread use for many energy or electricity generating plants. The present invention is designed to overcome these challenges.